Ion source



May 25, 1954 c. M. TURNER ION SOURCE 2 Sheets-Sheet l` Filed Jan. 28,1953 E] wwe/wwf' C/o/ence M Tar/7er May 25, 1954 M TURNER i 2,679,597

` 10N SOURCE Filed Jan. 28, 1953 2 Sheets-Sheet 2 e f M C/orence Turner*I m, 65M @iw Patented May 25, 1954 ION SOURCE Clarence M. Turner, StonyBrook, N. Y., assigner to the United States of America as represented bythe United States Atomic Energy Commission Application January Z8, 1953,vSerial No. 333,709

13 Claims. (Cl. Z50-41.9)

This invention is a continuation-in-part of the invention disclosed inapplication Serial No. 574,156, filed January 23, 1945, now abandoned,and relates to gaseous electric` discharge apparatus and especially toan electric arc discharge apparatus capable of producing in quantity asubstantially continuous supply of gaseous ions under vacuum. Moreparticularly, the electric arc discharge apparatus of the presentinvention is designed primarily for generating substantial quantities ofmetal ions, as are required, for example, in apparatus for separating orconcentrating the isotopes of metals such as copper 63 and 65, cobalt 59and 60, or uranium 234, 235, 237 and 238 by appropriate selectiveoperation upon ions of different atomic mass in an ion beam. Theseparation of such isotopes is important to provide quantities of aselected isotope for research, medicinal or industrial purposes such asradiography. This may be accomplished in accordance with this inventionby bombarding a vapor containing the several isotopes of the metal withthe electrons of an arc discharge established between noniilamentaryelectrodes one of which is thermionically emissive and maintained so bythe alternate electron bombardment of this electrode and anotherthermionically emissive electrode that results when these electrodes areconnected to an alternating voltage.

One type of isotope separating or ion-classifying apparatus in which theion source of the present invention may be advantageously used may befound described and claimed in a Patent 2,606,291 issued August 5, 1952,to Robert R. Wilson. This ion-classifying apparatus may consistessentially of an evacuated and sealed chamber, that may be cylindricaland of predetermined proportions, for supporting the ion source and itsion accelerating electrodes at an end thereof, and for supportingequipotential bunching electrodes at a predetermined distance spacedfrom said accelerating electrodes and maintained at the steady statepotential to provide a iield free space intermediate the acceleratingelectrode and the buncher structure for the flow therethrough of theaccelerated ionized particles. An alternating radio frequency potentialof small amplitude and preferably saw-tooth wave form is applied to thebunching electrode for periodically impressing increments of velocityboth positive and negative upon the ionized particles passingtherethrough to cause these particles to bunch together in accordancewith their mass. A radiofrequency alternating potential is applied alsoto a deiiecting electrode that is supported within the chamber at adistance spaced from said buncher electrodes to yprovide a substantialfield free space of definite length immediately following and adjoiningsaid bunch electrodes. This alternating potential is synchronized withthe buncher potential to deflect the grouped ions enriched with thedesired isotope of the ionized material into a collecting pocket or cuppositioned at or adjacent the end of said chamber.

That the operation of an ionic mass separating device such as thatdescribed above (or other types employing cooperating electric andmagnetic fields for classifying ions) may be eectively employed on aproduction basis it is essential that the ion source utilized in thedevice be capable of continuous operation for long periods withoutserious deterioration of the arc lelectrodes, to supply the ionsnecessary for large scale separation of the isotopes of the ionizedmaterial.

Prior to the present invention one effective procedure for producingions of a metal has been to bombard the vapors ofthe metal or ofsuitable volatile salts of the metal by the accelerated electrons of anarc discharge established between a lamentary cathode and an anode. Inthese procedures, particularly the procedures where metallic vapors aregenerated by depositing controlled quantities of the metal on the anode.the filament becomes contaminated with the source metal since it isexposed directly to the anode where the source metal is being vaporized.This contamination has in certain instances caused the filament to burnout after an operating time of only two hours.

Furthermore, when a fllamentary cathode is used with direct currentheating, the effect of the arc current flowing through the filament isto make one end hotter and the other end cooler, since in one end thefilament and arc current are in the same direction while in the otherend they are in opposite directions. This increase in heating of one endcauses the lilament to burn out more quickly than if the heating wereuniform. Attempts to eliminate this unequal heating by utilizing analternating current, for example, for heating the filament, have beenunsuccessful because the alternating current produced large fluctuationsin the ion output.

It has been discovered from observing the results of arc operation onthe lament that ode.

considerable amount of powermustbe-supplied to the filament to heat itto desired emission temperature. When direct heating of the cathode isused, as in the filament type of arction sources, the power requiredrepresents many hundreds of amperes preferablypdirect;current, at therelative low potentialo'f avolt or `two. Suitable low voltage sources"capable Jof sup-- plying this current magnitude present power supplyproblems that are :t-avoided 1in -this invention.

In the course of investigations undertaken rto extend the life of theelectrodes of an arc discharge capable Aof producing gaseous zions ,of"a material that' is normally'solid and vaporizable undervacuum4 onlywith the application .ofrhigh temperatures, it was discovered :that a.satisfactory operating timefor these arc electrodes,

greater by Aa factor of three,:than that iobtained formerly, could berealizedby utilizinganf'arc 'cathodesheated to electron emissivetemperatures byzthebombardment-thereof of accelerated electrons emittedfrom a lamentarycathode. .It

waszfound that a'platform cathode could'thus be' `utilizednand'heated`uniformly by .the electron vbombardment `from ,a heated filament -so`that the platform could be vpositioned intermediate theanodeand'filament to protect orlshieldvthe -filamentary .cathode from 'the'vapors within the varc region, thus'reducing ,the contamination offilament by .these vapors. IIt will be observed,

Aof course, 'thatthe unequal heating ofthe ila- ,ment vby ,the arci-andheating `current'is eliminated through the employment of such an;in

directly heated arc cathode.

`Ion' generatingiapparatus developed as a. result of this discoveryandof ya type `similar `to that described'and claimed in a copendingapplication,` S. Nul-371,981, filed May 241946,was found :alsoto'havezotherconcomitant advantages. .It

and -concomitant advantages possessed .by `this arc source of ionsemploying .the vindirectly heatedcathode,it'wasfound that this sourcefailed to avoid entirely the disadvantages attendant the use ofa heatedfilament, e. g. it was found .that after prolonged operation the `arcvcathode would eventually allow suflicient vapor to get to the heatingfilament vto establish an arc between this filament and the main cath-Furthermore, although alternating current could -be used to heat thefilament the necessity for a low voltage power source capable hundredamperes was `not avoided.

When 4these disadvantages were .noted to pergist, further researchleadingto the present invention was undertaken. It occurred to methat`the necessity for continuing the supply of heating current to thefilament could be dispensed with if the arc cathode could be heated byusing an alternating current between the filament and the arc cathode.Attempts to so heat the cathode by means of alternating current wereinitially unsuccessful however because the relectron bombardment of thefilament on the'reverse cycle `was suflicient tomelt the lament. Afterthe idea of utilizing two somewhat 1-10 rmassive non-lamentary cathodestructures for alternate bombardment with electrons was hit upon, andfound to avoid the previously encountered 'di1iculties; it appeareddesirable for =reasons of greater stability and longer life of '15 the'heating element, to attempt the development of an indirectly heated arccathode which 5would=preventat least for a prolonged period ofoperation, the penetration of the isotope metal vapors to the heatingelement; accordingly it was proposed to make the arc cathode @completelylcontain 'or enclose the intermediate or secondary cathode and to be sothick as to ,be substantially impenetrable.

-Thus :thearc electrodes of the ion source of the presentinvention weredeveloped and com- -prise specifically an'arc cathode constructed vofa'largerod of tungsten, for example, provided ywithan axial hole in oneend thereof for containing a second intermediate cathode also pref- .30erably of tungsten. A loop filament surrounds the arc cathode 4forproducing the electron ybombardment-for initially heating the arc cath-Iode to electron emissive'temperature, after which the filament isturned off and the heating maintained by connecting between the hollowlthimble- `likefarc cathode and the rod a source of alternatingpotential.

It was observed that, as a result of its smaller `mass, the intermediaterod cathode was heated .4U :to 1a higher temperature than .the outer andlarger arc cathode upon equal electron bombardment. lIt was discoveredthat .this inequality in heating could be eliminated by having thesource vof A. C. feeding the cathodes controlled by'thyratrons, or othergaseous discharge device f4" insuch a waythat the power fed into thethimble fand that fed into the1rod could be independently controlled.

.It `will be apparent from the foregoing there- 'fore vthatthisinvention has for its purpose the attainment Aof the followingobjects:

First, Vto provide an improved non-lamentary sourceof electrons for anarc discharge device;

Second, to provide an improved nonlamentary source of electrons having alarge emitting area for supplying a uniform and copious supply o'felectrons to an electric arc discharge device `thereby .enabling .the4arc to be started more fjeasily;

lfI'hird,..to provide an improved electronxsource v:for usein .an Varcdischarge device capable of Acontinuous `operation 'for long periods inthe presence of `corrosive vapors through which the -arc .discharge isestablished;

5 Fourth, to vprovide a non-lamentary source of electrons for an arcdischarge device that is operable on alternating currents at moderatevoltage magnitudes to avoid the disadvantages concomitant with theelectron sources operable of supplying continuously currents of Vseveral570 omyfrom 10W voltage Sources capable of sup- Vv arc discharge, isheated to electron emissive temperature as a result of mutual andalternate electron bombardment produced between it and another electrodespaced from the arc cathode and shielded from the region of the arcdischarge by being contained within said arc cathode;

Sixth, to provide a control means in a source of electrons for an arcdischarge device which will permit an independent control of theemission temperature of each of two spaced electrodes that aremaintained electron emissive by mutual and alternate electronbombardment on alternate half cycles of an alternating potentialsupplied to these electrodes.

A further and more specific object of this invention is to provide anefficient andl copious arc source of ions of a material that is normallysolid and vaporizable under vacuum only with the application of hightemperatures that employs an improved electron source for the arc thatis operable on alternating current and of increased life.

Further objects and many of the attendant advantages of this inventionwill be appreciated more fully as the same become better understood byreference to the following detailed description of a preferredembodiment, when taken with the accompanying sheets of drawing wherein:

Figure 1 is a detailed elevational View of a preferred embodiment of thepresent invention shown largely in cross-section the section being takenon lines I--I of Figure 2;

Figure 2 is a cross-sectional end view taken on lines II-II of Figure 1;and

Figure 3 is a Wiring diagram illustrating the operation of the presentinvention.

Referring now to the drawings and more particularly to Figures l and 2,the ion source illustrated is to be positioned within one end of anevacuated chamber housing the ion utilization apparatus, preferably bybeing supported within the insulated tubing I of quartz, porcelain, orother suitable refractory insulating material, to insulate the ionsource unit from the evacuated chamber so that it may be maintained at ahigh positive potential, of the order of 20,000 volts,

relative to an accelerating electrode II which isv indicated onlyschematically, to provide the electric eld for withdrawing the ions fromthe region of the arc and accelerating these ions to a high velocity.More specifically, the ion source electrodes are shown supported andsealed in a water cooled metallic end plate I2 in insulated relationtherewith, and this end plate is provided with a circular groove I2 intowhich the ceramic supporting cylinder Ill is inserted and sealed bymeans of the gasket ring I3 adapted to be cornpressed by an annularpacking gland, not shown, to maintain the air tight integrity of thechamber.

The end plate I2 is shown provided with a centrally disposed opening I4drilled therein for supporting the refractory insulating bushing I5 andthe concentric and spaced apart metallic tubes I6 and II that arepartitioned longitudinally for a substantial distance of their length toprovide a supply and return conduit for the circulation of a coolingfluid for cooling a chuck I8 that is shown inserted into the ends ofthese tubes and a cathode 20 which it supports. The chuck I8 has acentral opening I9 for receiving the thimble arc cathode 20 of tungstenor other suitable electron emissive material which is held in place byradially directed set screws of which 2| is representative. As an aid inthe support of the insulating bushing I5 and the concentric tubes I Eand I'I a cylindrical metallic sleeve 22` is shown welded to the outsidesurface of the end plate I2 concentric with the drilled opening I4. Theopening in the free end of this sleeve 22 is enlarged and tap threadedto receive the packing gland 23 for forcing a gasket material 24 intointimate contact with the insulating bushing to provide an airtight sealat the end of the bushing that projects through the end plate and to thehigh pressure end of this sleeve.

In the interests of simplicity of illustration no seal is provided forthe surface between the insulating bushing I5 and the outer metallictube I6 although it is apparent that in the eventi an airtight fitbetween the contacting surfaces of these members cannot be maintainedotherwise, a suitable sealing gland may bel easily installed.

An intermediate auxiliary or second rod cathode 25, also of tungsten, isshown supported to extend Within an opening 21 drilled in the end of themain arc cathode 20 preferably by means of the electrolytic drillingprocess in spaced and insulated relation with the arc cathode and itssupporting structure. Accordingly a chuck 26 for supporting the rod 25is shown supported coaxially within the tube I'I through a flangedinsulated bushing 28 and metallic sleeve 29.

More precisely, a pair of coaxial and spaced' apart tubes 30 and 3|support the chuck 26 Within the sleeve 29, to provide conduits forcirculating cooling fluid through the space 32 intermediate the tubes tocool the cathode rod 25 where it is supported in the chuck to preventwarping of this rod at its support. One effective l way of providing forthe flow of the cooling uid through the space 32 is to have the annularspace formed by the coaxial tubes partitioned circumferentially by meansof a pair of diametrically spaced wires having a diameter equal to theradial spacing of the tubes and of a length substantially equal to thetube lengths so that they will extend longitudinally down the space 32to terminate just short of the end of the tubes. Accordingly the tubes3|| and 3| are shown provided with entrant and exit tubes 33 and 34respectively for this cooling fluid.

Inasmuch as the space between the rod 25 and thimble cathode 20 must beevacuated if heating by electron bombardment is to be effective, thechuck I8 is shown provided with a plurality of radial openings 35 thatextend from this space to the exterior surface of the arc cathode tocommunicate with the chamber of the ion utilization apparatus that isconnected to a suitable pumping apparatus for continuous evacuation.

Furthermore, the high pressure ends of each pair of tubes I B, I1 and30, 3| are shown provided with packing glands 36, 31 for sealing theends of each surface of tubes I6 and 30, respectively, to the flangedinsulated bushing 28, and the metallic sleeve 29. Furthermore the highpressure ends of the tubes 30 and 3| are sealed olf as at 38. Tocomplete the insulation between thimble and rod cathode, and to securean inner airtight seal of the space between the metallic sleeve 29, andthe insulated bushing 28, as well as the space between the chuck I 8 andinsulated bushing 218, the chuck and flanged bushing are each shownprovided with circular recesses 4I and 42 formed in their end surfacefor receiving sealing rings 43, 44 and opposite ends of a glasscylindrical insulating sleeve 40. The insulated bushing 28 is providedwith a ange 45 against which the packing gland nut 36 is forced tosealithe glasssleeve' 40= betweenthe opposite sure facesfcontaining`thef recesses 4I .and .42 by com-V consisting cfa-tungsten wire 50encircling the.

thimble .cathode 20 and spaced radially a short distance from it, and atone side of the region. of the anode wherein the arc is established.Opposites` ends-ofthe-.,lamentary vstartingcathode 50 are. supported .bythe. lead-in conductors.

5I which are sealedzandinsulated in.:their pas.

sagefthrough'theend:plate-..I2;V by a bushing39.. of, suitablerefractory insulating.V material and. gland 49, shownin. cross-section.in-Figure 1, as: being similar in construction `to that employedIforsealing and insulating the tube-.IE-in the pas?- sagethrough theYend: plate- I2; It being understood thatfother'insulated seals-4 may beemployed:

and-also. thatan-.additionalfseal for each bushing 39. and conductor 5Imay-be provided if necessary to maintainthe chamber airtight. ".l'heYatitssupport towpreventwarping ofeither the` cathode orthe anode.

To feed metal tothearcas needed, an. appropriate supply,device.has-beenprovided. By Way of example, one suchdevicemay comprise a'feefly tube orduct-` 53 .extending thrQugnthe-tube 5d A which is -sealed andsupp0rtedtinthe .endplate I2 by means of the packinggland-55, said ductbeingshown .terminating-ata .point at .the .edge of and slightly. abovethe.platform 46. A wirei orribbon of metal 56 which maybecopper, ura i niumor cobalt .for example, mayv beadvanced continuously or in.l successivesteps` as. desired, through this -tube..53 .to the-.upper surface of theplatform. A.cooling.tube..51 is shown extending,- through thetube 54 incontact-With the feed tubel 53 .for the circulation therethrough. `of a`cooling iiuid for preventingr the` W-irefrom becomingheatedintheieedtube--to deleteriously effect the feeding operationthereof.

It .will beappreciated thatvarious-.supporting and feeding structurestobeamounted. inf-the end plate. I2 may include. suitable insulatingsealing or packing means (not shown) whereneeded, `in accordance withtheusualpractice for vacuum typeapparatus. Although mechanicalwire-advancing means, e. g. including a reel and .motor driven rollersdrawingthe wire. from the reel and pushing it through the tube53 (suchVparts being insulated. and enclosedv in a communieating evacuatedhousing if desired) may be employed in some cases to feed the wirecontinuously, Satisfactory` resultsv canbe obtained also by utilizing a.timing device set to operate the. feeding mechanism intermittently atthe desired time intervals.v

" cathode of the tube 6I.

Referring.. now4 to.Figure.3.fof` the. drawings. thereis..shown.in. thewiring diagram.one..conf. venient .way of yconnecting an. alternating,volt-I age to the two/cathodes -toproduce the alter.- nate electronbombardment on opposite half.V cycles thereof that includes also a meansWhereby the power input.into..each.of. the. cathodes may beindependently. controlled. It willV be. clear that the cathodes20. and25areconnected.

l* tothe A. C. supplyA source throughavariable.

autotransformer 60, for example, to provide .a control of the magnitudeof the applied. alter nating voltage. As shown,A the rod. cathode 251isfconnected directlyto the A. CL supplywhereas. the thimble cathode 20is connected. tothe A.l C. supply through a pair of gaseous dischargetubes 6I and62, oneofwhichisinverted. rela.- tive to the other toprovide` a currentconductive path onV alternate cycles. The discharge.

tube 62 is merely a rectier tube .connectedto provide a conductive pathfor ow of electrons: from the rod cathode 25 to the thimble arc cathode20 during the `portion of the cycle. of the alternating voltage appliedr thereto,` that I the thimble is positive relative to the rod.Theballast resistor 63 is provided merely to limit the current to a salevalue. The gaseous discharge tube 6I is grid controlled-and may berendered conductive at selective points in the cycle of-the alternatingvoltage when the rod 25 is positive relative to the thimble 20-byadjusting the phase of the grid and platel potential for example, tocontrol the heating of the rod 25 independently. of the heating of the.thimble 20. A suitable phase control circuit consisting of an adjustableseries resistance 65, and adjustable reactance 65 is provided and isshown energized from a secondary 61 of a transformer 68 in series with`a center-tapped resistor 69 connected to the The grid 'I0 of the tube 6Iis connected into the phase control circuitat the junction of the seriesconnected resistor 65 and reactor 66 through a resistance 'II vwhich isprovided merely to limit the grid current to a safe value after the tuberes.. This series resistor 1I may of course be omitted if bothimpedances 65 and 66 are at all times large enough to limit the gridcurrent. The phase of the grid voltage and time4 of firing of the tube6I is of course controlled byv changing the magnitude of the resistor 65or of the reactor 66 in accordance with well-known practice.

In operating the apparatus a switch S1 isv rst closed to connect thefilament of the starting cathode 5U to a source of heating voltage'I2ito supply 'I3 so that the thimble 20 is. at a positive electricalpotential relative to the filament to provide the accelerating voltagefor the thermionic emission of the cathode 50 to cause the electronsthereof to bombard the thimble to heat 1 the thimble to electronemissive temperatures.

After the thimble 20 is thus heated to a desired electron emissivetemperature, switch S4 may be closed to connect the thimble cathode 2G'and rod cathode 25 to the source of alternating voltage the amplitude ofwhich is controlled through the variable autotransformer 60, to causethe electrons of the thimble to bombard the rod to heat it also to anelectron. emissive temperature, after whichthe emission of each the.thimble.26.and rod 25 may be sustained by the'mutualbom- 9 bardment byelectrons of the tllimble and rod on alternate half cycles. The switchesSi and S3 are then opened to remove the source of heater voltage fromthe filament and to disconnect the filament from the source of electronaccelerating voltage. Switch S5 may then be closed, in the event thisconnection has not been previously established, to connect the arcvoltage between thimble arc cathode 20 and the platform anode 46. Theestablishment of this arc running to the anode upon which has beendeposited controlled amounts of the isotope source metal, results in thevaporization and subsequent ionization of this metal by electronbombardment of the vapors thus produced. The

CII

ions thus formed are withdrawn and accelerated in the form of a beamthrough a suitable accelerating electrode schematically indicated by thecylindrical shell Il by means of the high negative potential 13 to whichthis electrode is connected.

In View of the foregoing description, it will now be apparent that whatis illustrated and described is but a preferred embodiment of thisinvention that has been constructed and successfully operated whereinthe main arc cathode' comprises a tungsten thimble made byelectrolytically drilling a hole 1A; inch in diameter and 11/2 inch deepcentered in a tungsten rod 1/2 inch in diameter by 2 inches long; andwherein the intermediate cathode comprises a 15G-milA rod, also oftungsten which projects into this hole to within l/g inch of the end andserves as the heater for the arc cathode in the manner described above.It is conceived that this structure illustrated may be modified insubstantial meas" ure and applied to other uses without departing,

however, from the spirit of and scope of this invention as deiined inthe subjoined set of claims.

I claim:

1. In an arc source for producing ions an electronic discharge devicecomprising an anode, a first and second thermionically emissiveelectrode, starting means for initially heating at least one of saidelectrodes to an electron emissive temperature, means for applyinganalternating voltage between said first and second electrodes to causethe electrons emitted by an electrode rendered electron emissive by saidrst named means to bombard the other of said electrodes to heat saidother electrode for sustaining the emission of both of said electrodesby mutual electron bombardment thereof on alternate half cycles of saidalternating voltage and independently of said rst named means, means forproducing an arc discharge between one of said electrodes and said anodeand means for introducing into the region of said arc discharge a vaporof the material to be ionized.

2. The combination dened by claim 1 above characterized further by theaddition thereto of means for controlling the thermionic emission ofsaid electrodes relative to each other comprising means for controllingthe magnitude of said alternating voltage and means for selecting thepoint in the half cycle of said alternating voltage that the electronsemitted by said first one of said electrodes are made to bombard thesecond one of said electrodes, while maintaining throughout the completeother half cycle of a1- ternating voltage the bombardment of said rstelectrode by the electrons emitted by said second electrode.

3. An electric discharge device comprising a main hollow cathode, anauxiliary cathode lnsulatingly supported within the main electrode,

a gaseous medium disposed adjacent said main cathode, means including aiilametary element surrounding said main andl auxiliary cathode forrendering said main cathodes electron emissive, said auxiliary cathodebeing shielded from said iilamentary element by the main cathode meansincluding a source of alternating voltage connected between saidcathodes for producing an electric discharge running from said main tosaid auxiliary cathode to heat said auxiliary cathode to an electronemissive temperature, whereby the emission of said main cathode will bemaintained by the bombardment of electrons from said auxiliary cathodeon alternate half cycles of said alternating current independently ofsaid lainentary means, and means for directing electrons emitted fromsaid main cathode into said medium, whereby to ionize said medium.

4. The combination defined in claim 3 above characterized further by theaddition thereto of means in circuit with said auxiliary cathode forselecting the portion of the alternating voltage cycle that theelectrons of said main cathode bombard said auxiliary cathode whilemaintaining the bombardment of said main cathode by the electrons ofsaid auxiliary cathode throughout said half cycle of said alternatingvoltage that the main cathode is positive relative to said auxiliarycathode.

5. An electric discharge device comprising a first cathode, an anode, asecond cathode disposed intermediate said first cathode and said anode,means for rendering one of said cathodes electron emissive, means forapplying an alterhating voltage between said first cathode and saidsecond cathode whereby electrons emitted `from the cathode renderedelectron emissive bombard the other of said cathodes to raise said othercathode to an electron emissive temperature, means for producing a vaporof the material to be ionized in the region disposed between said secondcathode and said anode and means for producing an arc discharge betweensaid second cathode and said anode through said vapor containing regionto ionize said vapor.

6. A11 electric discharge device comprising an evacuated sealed chamber,means for introducing a gaseous medium to a region within said chamber,a hollow main cathode supported adjacent said region, an auxiliarycathode completely contained within said hollow cathode to be shieldedthereby from said gaseous medium, means for initially rendering one ofsaid cathodes electron emissive, alternating current means forsustaining said electron emission of both said cathodes independently ofsaid first named means by the mutual electron bombardment thereof, andmeans for directing into said region electrons emitted by said maincathode whereby gas in said region is ionized.

7. The combination deiined in claim 6 above characterized further by theaddition thereto of means for controlling the electron bombardment ofsaid cathodes relative to each other to control the degree of heatingthereof.

8. An electric discharge device comprising an evacuated sealed chamber,means for introducing a gaseous medium into a region within saidchamber, a hollow rod arc cathode, an auxiliary rod cathode disposedtherein, a iilamentary cathode spaced radially from and surrounding saidhollow rod cathode, means including a source of electric power forraising the temperature of said lamentary cathode to render it electronemissve, Ameans including a `voltage source for bombarding 'said'hollow'rodcathode with elec- 'trons emitted 'from `sai`d "lamentarycathode, 'whereby said hollowV rod cathode is rendered electronemissive, vmeans including a source of alter- 'nating voltage connectedbetween said hollow rod arc cathode and said'auxiliary rod cathode forproducing an electrcdischarge from said arccathode to said`auxiliarycathode whereby vsaid'auxiliary catho'deis' rendered electronemissive'for sustaining the 'electron emission of both 'said .arccathode and auxiliary cathode independently of said`la'ment` by themutual electron` bombardment thereof on alternate half cycles ofsaidalt'ernating voltage, and means 'for directing electrons'emitte'd bysaid arc cathode into said region to ionze 'said gaseous medium. 9. Anelectric .discharge device comprising a .pair of'thermionicallyemissive'electrodes, means ttor-initiating thermionic emission'from atleast "one ofsaidelectrodes, means including a source 'of alternatingvoltage between said electro'desor 'causing the'electrons emitted by oneelectrode to bombard`the other of said electrodes to heatsaidotherelectrode and sustain by mutual electron bombardment' theemission of both of said electrodes. and a grid`controlled gaseouselectric ldischarge'fdeviceseries connected between one of said'lelectrodes' and said 'source for controlling the portion of the halfcycle ofelectron bombardmentof one' of saidelectrodes, whereby the powerinputto' said one of said electrodes may be con- `trolled`independentlyof said other electrode.

10. AIn an are sourceorjproducing ions, an

electronic'discharge device comprising first and 1 secondele'ctrodes,means forinitiating' thermionic "emission vfrom'at least'one of saidelectrodes 'and `means for "sustaining the emission thereof by mutualelectron' bombardment' of said. electrodes,

lf2 "meanssupporting' one .of' saidelectro'desrendered -electronemissive within a, 'region 'containing ya gaseous medium, means for'directing electrons from one'said electrodeintosaidregion whereby saidgas insaid' region is ionized,Y and means-for controlling :the electronbombardment of :said electrodes relative' to "each 'other Nto' controlthe degree of heating thereof.

' 11. 'An electronic discharge 'device-'comprising a "pair "ofthermionic'ally "emissive electrodes, "a gaseous "medium disposedAadjacent 4one "of said electrodesfmeans including :a'source of^alternating'voltage connectedV betwen saidpelectro'des'for rendering`said electrodes electronic lemiss'rva meansfor'directingelectronsemitted by saidone electrode into 'saidgaseousmedium whereby'said "gasis ionized, "and :means Jforwzontrolling uthe 'lthermionic "emission'temperature of 'said electrodes relative tov each other.

'12. An `ion generating "device comprising "a .sealed chamber, arelatively massive cathode 'withinsaidnhambenan auxiliary vcathode with-;"in said'main cathode and completely 'shielded "thereby,"'means'forinitiallyrendering oneof said cathodes electron `emissive, alternating'current means connectedto said cathodes for sustaining saidelectroniemissionindependently of' saidvrst :means 'by` the -mutualn'electronic bombardment w thereof an anodespaced'ffrom 'saidmaincatho'del andmea'ns forstrikln'g an arcf"discharge"be 'tween said' arc`cathode and' `saicl'anode.

13. Thestructure Lofclaim 12 including means "forintroducing amaterialto'be'vaporzed and ionizedwithin said sealed chamber and saidanode beingv in'the form of a" platform positioned to 'shield' said'mancathode'from said materiales "it' 'vap'orizes 'Norreferencescited

